A number of imaging systems are known including CT scanning, PET (Positron Emission Tomography) and MRI (Magnetic Resonance Imaging) which can be used to image the body.
With MRI, a high field magnet, typically superconducting, is arranged in a torus configuration (like a donut) and with the patient lying down inside the magnet the magnetic field allows a pulsed and sequenced magnetic and EM field to probe the body to produce soft tissue images, which allow the trained radiologist to determine with high probability the anatomy of the patient. MRI is sometimes performed using contrast agents to provide even better contrast between different soft tissue types. MRI techniques are very good at detecting the anatomical location of many but not all tumours.
These two imaging techniques, MRI and PET, and MRI and CT are orthogonal techniques, in that the detection technology can be constructed in such a way that it is unaffected by magnetic fields and the MRI system can be constructed to be unaffected by the other techniques. For this reason, if an integrated detector system can be constructed, then the two systems can be operated in parallel or almost parallel manners in both space and time to allow for improved and more complete detection of tumour locations within the body.
In order to carry out these dual imaging systems it is often necessary to move components of the system relative to the magnet. These may include the imaging systems themselves or components associated with the system such as a patient support or table.
Further, previous publications and patents have shown it is possible to move a high-field superconducting MRI system in translation.
In U.S. Pat. No. 5,735,278 (Hoult et al) issued Apr. 7 1998 is disclosed a medical procedure where a magnet is movable relative to a patient and relative to other components of the system. The moving magnet system allows intra-operative MRI imaging to occur more easily in neurosurgery patients, and has additional applications for liver, breast, spine and cardiac surgery patients. The magnet is at first some distance from the operating table, either in the side or back of the surgical room or perhaps within a holding bay area.
When imaging is required, the MRI magnet is advanced from its holding area and positioned in the imaging position over the table, images are taken and magnet is retracted to its holding area.
Thus the MRI system consists of the magnet; rails, installed on the site; and a magnet mover system, which consists of a magnet carriage, cable carrier, and mover control system.
In order for an MRI system to generate effective images it is necessary that the magnetic field within the target area is homogeneous. Adjustments to the system to achieve this homogeneity are known as shimming.
The computer system controlling imaging has an internal control protocol which carries out automatic fine tuning of the shimming. However this automatic control has limits of adjustability and it is necessary when installing the system to firstly carry out an installation shimming. Thus it is known that an MRI magnet is shimmed, relative to its location and the arrangement of magnetic material in the area around the magnet, by adding small pieces of magnetic material in very precise positions in between the gradient and the gradient shield. This passive shimming is complemented by resistive shimming which can be achieved by placing coils of the appropriate geometry immersed in liquid helium in the magnet as well as on the outside of the gradient assembly. The coils immersed in liquid helium are called cryo-shims and those attached to the gradients dynamic resistive shims. The homogeneity of the magnet can also be adjusted by placing constant current in the gradient coils and these are known as the gradient offset currents. The passive and cryo-shimming is carried out only one time for a particular site during installation. This is carried out by introducing a “shim rig” into the bore of the magnet which includes sensors responsive to the magnetic field at the locations in the bore and after that the automatic control protocol of the imaging system is relied upon to effect the fine adjustment necessary.
Up to now, therefore careful attention has been applied to maintaining the environment of the magnet constant so that there is no changes which would require re-shimming of the magnet.